If I wanted to HQ my synths, would I be ok hitting a HQ tier only for the main craft (in the example case, Alchemy) or would I need to hit that HQ tier with my subs as well (in the example case, Goldsmithing)?

So, say you were 31 levels over on Alchemy but only 11 levels over on the goldsmithing sub, you can expect T1 HQ results only.

This is why in the early days of crafting, people were trying so hard to figure out (for example) the subcraft level for leather on the scorpion harness recipe - so they could tweak their skills to ensure 31 levels over for both main and sub, thereby achieving T2 results.